Interaction of embryonic surface and cytoskeleton with extracellular matrix

Evidence for cell-matrix in vitro and in the embryo is briefly reviewed, and more detailed observations are presented on the reactions of corneal epithelium and mesenchyme to extracellular matrix (ECM). The basal surface of embryonic corneal epithelium blebs when the underlying ECM is removed. If the epithelium is cultured on top of the lens capsule or collagen gel, the basal surface flattens and the cortical cytoskeleton reorganizes to resemble that present in vivo. The basal surface also responds to soluble matrix molecules (types I-IV collagens, laminin, fibronectin), and the cells step up synthesis of corneal stroma as measured by incorporation of proline into collagen. When embryonic corneal fibroblasts are placed on top of hydrated gels they tend to burrow into the gel rather than sitting on top as does epithelium. When grown inside collagen gels, these mesenchymal cells elongate and the entire cell surface and cytoskeleton organize in response to matrix. Stress fibers and ruffling membranes characterize the cells grown on glass. When embryonic lens or corneal epithelial are place within, instead of on top of, collagen gels, they give rise to mesenchyme-like cells from their apical surfaces. In vivo, these epithelia do not give rise to mesenchyme. The rules for epithelial-mesenchymal transformation in vivo are discussed in relation to these observations on cell-matrix interaction.